Unraveling mechanism of the 2021-2022 South Alboran Seismic Swarm: Insights into Fault Reactivation and Lithospheric Dynamics

Abstract

The 2021-2022 seismic swarm in the southern tip of the Alboran Shear Zone provides a unique opportunity to explore the interplay between tectonic, fluid-driven, and possibly mantle-related processes within the tectonically complex western Mediterranean. The swarm lasted more than 2 years and is characterized by low magnitudes up to M5 and the lack of a dominant mainshock. The relocation with HypoDD revealed a distinct spatial clustering and temporal migration patterns. The activity is concentrated along the southern termination of the Al Idrisi Fault Zone, a left-lateral strike-slip system, and the Trougout Fault, a normal-left lateral strike-slip fault, as major onshore-offshore structures. Focal mechanism analysis reveals predominant strike-slip and transtensive faulting, consistent with the left-lateral motion and the ongoing convergence between African and Eurasian plates governing deformation along the Alboran Ridge. Furthermore, the seismicity reaching depths of 30–35 km points to deeper lithospheric processes, suggesting fluid migration, mantle dynamics, or crustal densification, as potential triggers for fault reactivation. Our analysis of the distance-time (R-T) diagrams indicates that the swarm is likely fluid-driven supporting the hypothesis of fluid migration as a key factor in fault reactivation. The localization of this activity along a shear zone highlights the role of distributed deformation and stress redistribution in regions dominated by transpressional and transtensional regimes. The absence of large seismic events suggests that the fault system may accommodate stress through slow-slip events or fault creep, where deformation occurs gradually rather than through abrupt ruptures. This mechanism is often observed in weak or immature fault zones, where strain accumulates and releases in smaller, episodic bursts instead of a single catastrophic failure. Such behavior is typical of regions undergoing lithospheric thinning or mantle upwelling, where thermal anomalies and stress perturbations influence fault dynamics.